Cellulose esters are valuable polymers which are useful in many plastic, film, coating, and fiber applications. In coatings applications, cellulose esters are typically applied from organic solvent solutions, although those having sufficient carboxyl content may be dispersed in water, with the aid of organic solvents, and applied as an aqueous dispersion or solution.
A variety of carboxylated cellulose esters are known, and may be used in coating compositions. For example, carboxyalkyl cellulose esters, such as such as those described in U.S. Pat. Nos. 5,668,273, 5,792,856, and 5,994,530, the disclosures of which are incorporated herein by reference in their entirety, are soluble in a variety of organic solvents. As described in Eastman Chemical Company Publication No. GN-431A (February 2003) entitled “CMCAB 641-0.2 and 641-0.5 Cellulose Esters,” these esters can be dispersed into water by preparing an organic solvent solution of the carboxyalkyl cellulose ester, and adding to the solution either ammonia or an amine, and water. The water can be added separately from the ammonia or amine, or can be premixed with the ammonia or amine. The resulting dispersions typically have a solids content from about 5 to about 25%, and the ratio of cellulose ester to solvent ranges from about 1:4 to about 1:1. Because this method requires at least a 1:1 ratio of ester to organic solvent, the resulting dispersions have a relatively high volatile organic compound (VOC) content. Environmental regulations limit the amount of VOC content in certain coating formulations, and further such restrictions are expected. There remains a need in the art to provide aqueous carboxylated cellulose ester dispersions having a lower VOC content.
U.S. Pat. Nos. 5,668,273 and 5,792,856 describe certain carboxyalkyl cellulose esters of higher acids, and especially carboxymethyl cellulose esters of higher acids, such as carboxymethyl cellulose acetate propionate, carboxymethyl cellulose propionate, carboxymethyl cellulose acetate butyrate, and carboxymethyl cellulose butyrate. The ether derivatives of the cellulose esters described exhibit solubility in a wide range of organic solvents, compatibility with a variety of resins, and ease of dispersion in a waterborne formulation. The aqueous dispersions described in these patents are prepared by dissolving the esters in organic solvents, partially neutralizing the esters, and dispersing them in water. While the resulting aqueous dispersions are suitable for use in a wide variety of coating formulations, their high VOC content makes them unsuitable for applications requiring a low VOC content.
These patents also describe zero VOC dispersions in aqueous ammonia that are quite water tolerant. The dispersions described are, in effect, ester solutions, since the ester is essentially solubilized in the ammonia. Because of the water tolerance of these ester solutions, they may be water-diluted to form what are, in effect, fully neutralized aqueous solutions. However, because of the quantity of ammonia required to solubilize the esters, the pH of these solutions is typically very high, which can compromise the hydrolytic stability of the cellulose esters.
Similarly, U.S. Pat. No. 5,994,530 describes carboxyalkyl cellulose esters of higher acids, and their use in aqueous dispersions useful in formulating waterborne coating compositions containing pigments, especially metallic pigments such as aluminum flakes and mica. These aqueous pigment dispersions may be prepared by adding an organic solvent solution of the ester to a metallic dispersion, partially or fully neutralizing the ester (from about 25 to about 100% of the pendant carboxyl groups) by adding an organic or inorganic base, and then mixing in water. In alternative methods, the base may be added with water prior to the addition of organic solvent, or added to an organic solution of the ester, either undiluted or diluted with an organic solvent. The resulting mixture may then be added directly to an aluminum flake dispersion containing only aluminum flake and an organic solvent. In further alternative methods, the ester may be added directly to aluminum flake which contains a solvent that will dissolve the ester, or the ester may be first swelled with a suitable aqueous base, followed by addition of a suitable organic solvent. While these methods provide aqueous ester dispersions that provide excellent metallic flake orientation properties, the VOC content may be higher than applicable environmental regulations allow.
U.S. Pat. No. 5,319,019 describes aqueous dispersions of an ammonia salt of an acrylic polymer having a low VOC content. A method is disclosed in which an amine salt of a high molecular weight acrylic polymer is prepared in a water-immiscible solvent. Water is then added to this solution to invert the mixture to form a salt/solvent azeotrope, and the azeotrope is then heated to remove the organic solvent, providing an aqueous dispersion of the polymer having less than 2 weight % organic solvent content.
In another process disclosed in the '019 patent, water is added to an organic solvent dispersion (consisting of ethylene glycol monobutyl ether alone, or a mixture of the ether with an organic solvent that is substantially immiscible in water) of the acrylic polymer, or the ammonia salt of the acrylic polymer, to form a mixture containing the ammonium salt of the acrylic polymer, organic solvent, and water. This mixture is inverted by the addition of water to form an organic-solvent-in-water azeotrope, having a boiling point from 95° to about 99° C. This azeotrope is heated to remove the organic solvent, and the water and solvent are condensed as they are removed from the dispersion. Xylene is added to the condensed water and solvent to effect a separation of the water and solvent, and the condensed water is circulated back into the mixture, to provide an aqueous dispersion of the ammonia salt of the acrylic polymer having less than 2 weight percent organic solvent. The patent also mentions that amines may be used in place of ammonia.
U.S. Pat. No. 5,356,988 describes aqueous dispersions and processes for making them, in which the salt of an acrylic polymer is prepared that is the reaction product of an acrylic polymer, an organic solvent which is either an organic solvent which is substantially water-immiscible or ethylene glycol monobutyl ether (which is water miscible at 25° C.), and a salification agent which is an amine or ammonia. According to the process described in the '988 patent, the aqueous dispersion is formed by adding the salt mixture to water at a rate of not more than about one-fourth of the salt/organic solvent dispersion per hour, the rate of addition being a function of the rate of the removal of the organic solvent through the heating or distillation of the azeotrope. The patent advises that at all times during the addition, no more than 15 weight percent of the organic solvent, based upon the total weight of organic solvent used to disperse the acrylic polymer, should be in the heated water/organic solvent mixture, in order to prevent the viscosity from increasing to unacceptably high levels. The higher the ratio of solvent to acrylic polymer or salt thereof, the more solvent which has to be azeotroped to be removed and discarded, and the lower the ratio of solvent to acrylic the greater the difficulty of dispensing the polymer in water will be. In an alternative embodiment, the polymer may be dispersed in the organic solvent and added as described above to water which includes amine or ammonia in an amount effective to neutralize the polymer.
Similarly, U.S. Pat. No. 5,380,771 describes a process in which a dispersion of substantially water-immiscible organic solvent and a water dispersible amine salt of a high molecular weight acrylic polymer are mixed with water, the mixture is inverted, and the organic solvent removed by heating or distilling the inverted mixture of the water dispersible acrylic amine salt, an azeotrope being formed from the water and the substantially water immiscible organic solvent. The azeotrope is said to have a boiling point of not more than 95° C., and the substantially water immiscible organic solvent a boiling point of at least above about 100° C. The amount of water in the amine salt/organic solvent/water mixture during the heating or distilling of the azeotrope should be maintained in an amount effective to provide a solids level in the final aqueous dispersion that is desired.
U.S. Pat. No. 6,046,259 describes in a comparative example the preparation of a cellulose acetate butyrate aqueous dispersion in which the cellulose ester is dissolved in methyl isobutyl ketone, and surfactant and water then added, followed by agitation at room temperature for 15 minutes. The mixture is then passed through an emulsifier to produce a microdispersion, which is then heated for azeotropic distillation of the ketone solvent.
U.S. Pat. No. 6,225,402 discloses neutralizing in an organic solvent an acrylic-modified chlorinated polyolefin polymer having ionizable groups; blending the solution of neutralized polymer with water to form a blend of neutralized polymer, organic solvent, and water; and removing the organic solvent from the blend at a temperature and pressure effective for forming a dispersion of the acrylic-modified chlorinated polyolefin polymer.
U.S. Pat. No. 6,277,953 describes stable aqueous polymer dispersions and processes for their preparation. According to the '953 patent, an ionic functional polymer is synthesized in a hydrophilic organic solvent, or synthesized neat and subsequently mixed with organic hydrophilic solvent. An amount of neutralizer is then added that is effective for providing a dispersion of the polymer upon addition of water and removal of solvent. After the formation of the polymer salt solution in the hydrophilic organic solvent, and mixing that solution with water to form a water/organic solvent/polymer salt blend, the organic solvent and water are removed or stripped from the blend at a duration, temperature, and pressure effective for providing an aqueous dispersion having a resin mean particle size of not greater than about 300 nm, a polymer concentration of at least about 30 weight percent and an organic solvent concentration of less than 2 weight percent.
U.S. Pat. No. 6,306,956 describes an aqueous dispersion of a polyester salt. According to the '956 patent, an ionic functional polyester polymer is synthesized neat and subsequently mixed with a hydrophilic organic solvent. The polyester has sufficient ionizable groups to provide the polyester with an acid value of from about 30 to about 50 and a solubility in the hydrophilic solvent of at least about 50 weight percent; and the organic hydrophilic solvent has a solubility in water of at least about 5 weight percent. At least about 30 percent of the ionizable groups of the polyester polymer in the solvent are neutralized with an amount of neutralizer effective to form a solution of neutralized polymer salt. The solution is thereafter mixed with water to form a blend of water/organic solvent/neutralized polymer. The organic solvent is then stripped from this blend at a temperature no greater than 65° C. to form a dispersion of neutralized polymer salt in water.
U.S. Pat. Appln. Publn. No. 2002/0028874 A1 discloses stable aqueous polymer dispersions and a process for their preparation. According to this publication, an ionic functional polymer is synthesized in a hydrophilic organic solvent, or synthesized neat and subsequently mixed with organic hydrophilic solvent. After the polymer is fully dissolved, an amount of neutralizer is added effective for providing a dispersion of the polymer upon addition of water and removal of solvent. After formation of the polymer salt solution in the hydrophilic organic solvent, and mixing that solution with water to form a water/organic solvent/polymer salt blend, the organic solvent and water are removed or stripped from the blend at a duration, temperature and pressure effective for providing an aqueous dispersion having a resin mean particle size of not greater than about 400 nm, a polymer concentration of at least about 30 weight percent and an organic solvent concentration of less than 5 weight percent.
Similarly, WO 00/18826 and WO 02/10297 disclose stable aqueous polymer dispersions made by processes similar to those already described.
There remains a need in the art for a method of dispersing carboxylated cellulose esters in water that results in an aqueous dispersion that is suitable for use in a variety of waterborne coating compositions, while having a relatively low VOC content.